The present invention generally relates to cellular and wireless communication. More specifically, the invention relates to a method for making neighbor cell measurements in a cellular communication system employing a discontinuous control channel carrier. The present invention also relates to mobile stations that perform such neighbor cell measurements.
Recently, there has been a trend in the telecommunication community to focus more and more on wireless packet data communication rather than circuit switched voice communication. With the tremendous increase of Internet users, it is believed that the packet switched communication will soon increase more and become larger than the circuit switched voice communication that today dominates, e.g., the cellular communication. Cellular communication system manufacturers and operators are therefore looking for solutions to integrate their circuit switched services with wireless packet switched services that can provide reliable and more spectrum efficient connections for packet switched users, e.g., Internet users. This trend has made different types of packet switched communication system evolutions flourish. One of the more well known packet switched cellular systems in the telecommunications community, is the extension of the present Global System for Mobile Communication (GSM), known as General Packet Radio Service (GPRS).
GPRS is a packet switched system that uses the same physical carrier structure as the present GSM cellular communication system and is designed to coexist and provide the same coverage as GSM. The GPRS radio interface is thus based on a TDMA (Time Division Multiple Access) structured system with 200 kHz carriers divided into eight timeslots with GMSK (Gaussian Minimum Shift Keying) modulation. The multiplexing is such that each timeslot can typically serve a number of users. One user can also be allocated more than one timeslot to increase its throughput of data over the air.
The GPRS specification includes a number of different coding schemes to be used dependent on the quality of the radio carrier. With GPRS, data rates well over 100 kbps will be possible.
There is also ongoing a development and standardization of a new air interface mode in GSM, which will affect both packet and circuit switched modes. This new air interface mode is called EDGE, Enhanced data rates for global evolution. EDGE""s main features are new modulation and coding schemes for both packet switched and circuit switched data communication. In addition to the Gaussian Minimum Shift Keying (GMSK) modulation, an 8 symbol Phase Shift Keying (8PSK) modulation is introduced. This modulation can provide users with higher bit rates than GMSK in good radio environments.
A new technique called link quality control is introduced with EDGE. Link quality control is a functionality that allows adaptation in terms of coding and modulation with respect to present signal quality. In poor radio conditions, a robust coding and GMSK modulation is selected whereas in good radio conditions, a less robust coding and 8PSK modulation is used. GPRS (and the extensions thereof) also provides a backward error correction functionality in that it can request retransmissions of erroneously received blocks. This mechanism is called ARQ (Automatic Repeat reQuest) and is well known in the art.
The packet data mode with EDGE modulation is called EGPRS (Enhanced GPRS) and the circuit switched data mode is called ECSD, Enhanced Circuit Switched Data. Bitrates over 384 kbps will be possible with EDGE.
Recent development for another TDMA based cellular system, the cellular communication system compliant to the ANSI/136 standard, below referred to as TDMA/136 has been focused on a packet data system to be integrated with the TDMA/136 circuit switched mode.
This packet data system will also be based on the new EDGE technology as defined for the GPRS extension. It will then allow TDMA/136 operators to provide bit rates up to 384 kbps on 200 kHz carriers with GMSK and 8PSK modulation as defined for EGPRS.
This integration of TDMA/136 and EDGE, does not, however, come without a cost. The TDMA/136 carriers have a bandwidth of only 30 kHz, to be compared with EDGE carriers of 200 kHz. This means that operators that want to introduce EDGE, have to allocate 200 kHz for each EDGE carrier or, to put it in another way, to free up spectrum for each EDGE carrier corresponding to 7 already existing 30 kHz carriers. Since operators already today are using these 30 kHz carriers for circuit switched communications, there is a large interest that the initial deployment for EDGE in TDMA/136 systems should be made on as small a spectrum as possible.
Reuse patterns are used in cellular systems, such that one can reuse the same frequencies in different cells. Systems are usually planned such that a number of cells share a number of available channels. For example, in a 4/12 frequency reuse, there are 12 different cells that share a set of frequencies. Within these 4/12 cells, no frequency is used in more than one cell simultaneously. (The number 4 in xe2x80x9c4/12xe2x80x9d denotes the number of base station sites involved in the 12 reuse. The 4/12 denotation thus indicates that a base station site serves 3 cells.) These 4/12 cells then form what is referred to as a cluster. Clusters are then repeated, to provide coverage in a certain area.
Similarly in a 1/3 reuse, there are 3 different cells that share a set of frequencies. Within these 3 cells, no frequency is used in more than one cell simultaneously. Thus, the higher the reuse, the better the carrier to interference ratio for an exemplary condition. For lower reuse patterns, interference is the carrier to interference ratio is lower, since the distance between two base stations transmitting on the same frequency is shorter. An exemplary 1/3 reuse is illustrated in FIG. 1.
GPRS channels typically have different levels of robustness depending on the type of logical channel being transmitted. A logical channel is defined by its information content and is transmitted on one or several physical channels, defined by the physical channel structure, e.g., a timeslot on a certain frequency. In a packet data system, reliance on retransmission possibilities can allow a quite high error rate which means that the reuse for user data traffic channels can be kept quite low. For example, a data traffic channel can be deployed in a 1/3 reuse whereas common control channels and broadcast channels are not robust enough to be allocated in a 1/3 reuse, since the same retransmission possibilities are not used for these types of logical channels. At least a 3/9 or even a 4/12 reuse is recommended for packet data common control and broadcast channels.
Note that a 3/9 reuse entails that at least nine 200 kHz carriers are needed (i.e., TDMA operators must provide at least 1.8 MHz of spectrum for an initial deployment). This is considered quite substantial in a TDMA system with 30 kHz carriers.
This fact has driven the TDMA community to find other solutions for initial deployment of a packet data system based on EDGE and GPRS. U.S. patent application Ser. No. 09/263,950, xe2x80x9cHigh Speed Data Communication System and Methodxe2x80x9d, to Mazur et al., hereby incorporated by reference, teaches a method of combining TDMA/136 and the EGPRS mode of EDGE.
Briefly, the solution is to put requirements on the base station transmissions of the EDGE carriers. Base station transmissions of EDGE carriers should be time synchronized. It is then possible to allocate the control channels on different frequencies and different timeslots in different cells and thereby construct a higher reuse than what is possible by only considering frequencies. This solution is often referred to as EDGE Compact. In addition to the frequency reuse, a time reuse is introduced. For example, a certain base station transmits control signalling on a certain timeslot at a certain time and on a certain frequency, at which no other base station in the same control channel cluster (i.e., all cells where each physical channel carrying control signalling is used once and only once) is transmitting anything at all. This is repeated between a number of base stations, such that different time groups are formed. Further, to increase reliability of control channel detection in the mobile stations and base stations respectively, timeslots adjacent to each other do not both carry control channel information.
EDGE Compact provides the opportunity to introduce a higher reuse than that allowed by frequency repetition only. Thus, it will be possible to allow an initial deployment of a GPRS/EGPRS packet data system within a spectrum bandwidth much smaller than that otherwise limited by the reuse requirement for the control channels. In FIG. 4, a typical allocation for the control channels is illustrated. Therein, four different time groups are illustrated on a single frequency, i.e., a 4x time reuse is formed. In one cell, control information is transmitted in timeslot 1, (TS 1), i.e., timegroup 1 (TG1), in certain GSM frames defined. Base stations transmitting control information on the same frequency but belonging to another time group, will not transmit at all during the frames that are used for control in base stations belonging to TG1. In another cell, control information is transmitted in TS3 (i.e., TG2), again in certain GSM frames. Base stations transmitting control information on the same frequency but belonging to another time group, will not transmit at all during the frames that are used for control in base stations belonging to time group 2. Similar reasoning applies for TS5 and TS7. Combining the time reuse with, e.g., a 1/3 frequency reuse, it is possible to transmit control information in an effective 4/12 reuse using only 3 frequencies. In FIG. 4, different types of control, information or logical control channels have been indicated. In block B0, broadcast information is transmitted on a logical Broadcast Channel (BCCH) and, e.g., in block C8 logical Common Control Channels (CCCH) are transmitted (e.g., paging messages). The structure of the control channel is such that more blocks than those indicated can be allocated for broadcast or control. For example, if one more block is needed for CCCH, this can be allocated in physical block 2, on GSM frames 8-11. Allocation of 2-12 blocks is possible on a single timeslot. One broadcast information block and one common control block is always needed.
Further, to be able to find this control channel, a frequency correction burst and a synchronization burst is included in each 52 multiframe. A mobile will first search for the frequency correction burst (located in GSM frame 25), and it will know that following this, there will be a synchronization burst 26 GSM frames later, on the same timeslot. This synchronization burst helps the mobile station identify the base station and learn where in the multiframe structure it is.
FIG. 3 illustrates an exemplary cell pattern that is formed of the reuse of time groups and frequencies combined. Note that in EDGE Compact, only the control channels are transmitted in the higher reuse, utilizing the time groups. The traffic channels are still transmitted in a 1/3 reuse.
EDGE Compact, will provide the possibility to deploy a packet data system in a spectrum well below the 1.8 MHz. In the example described, operators may deploy an EDGE Compact system with only three 200 kHz carriers.
As mentioned, the transmission of control information in EDGE Compact is different than the control channel transmissions in present GSM systems. Present GSM systems have at least one carrier in each cell that transmits continuously with constant power (i.e., it transmits on all timeslots, even if there is no traffic allocated). In present GSM systems, this continuous transmission serves as a beacon in the system, for mobiles to more easily find the control channel carrier, identify the cell and, e.g., make signal strength measurements for Mobile Assisted HandOver (MAHO) algorithms.
For MAHO, mobile stations report to the network how well they can hear neighbor cells, and what signal strengths they perceive. Then, based on those measurements, more reliable handovers are possible, as mobiles move between different cells.
In the EDGE Compact system, control channel signaling is only transmitted during one timeslot in each GSM frame. Signal strength measurements should be made on the control channel transmissions rather than on traffic channels, since traffic channels can be power controlled (i.e., transmitted with varying power). Thus, a mobile must know exactly in what timeslot it should measure on neighbor cells. This requirement has not existed with the earlier continuously transmitted control channel carriers.
Thus, a new strategy is needed for neighbor cell measurements in communication systems such as EDGE Compact. More specifically, there is a need for a technique which ensures that a mobile station measurement window opens when control channel signaling is actually transmitted from a neighbor.
In one aspect of the present invention, a method is provided for a mobile to make reliable signal strength measurements on neighbor cell control channel transmissions allocated on a carrier frequency that is not transmitting continuously.
This method contains the steps of defining a measurement window that does not extend longer in time than the shortest period at which a control channel transmission continuously extends in duration, for example a timeslot duration. The method further includes the step of opening a measurement window in the mobile at an appropriate time and closing the measurement window at an appropriate time, to ensure that the mobile actually measures on the control channel transmissions transmitted from base stations in neighbor cells. The appropriate time is calculated in the mobile station based on information as to which timeslot is used for control signalling from a certain neighbor base station, as well as information about the timeslot reference in the base station serving the mobile station.
In another aspect of the invention, a propagation delay estimate is performed in the mobile station, such that propagation delays from a neighbor cell base station are taken into consideration when a calculation of appropriate measurement times is performed.
In another aspect of the present invention, a measurement functionality is included in mobile stations so that each mobile station can calculate an appropriate measurement time based on input parameters including information about a timeslot reference in a serving cell and information as to which timeslot a neighbor base station uses to transmit control information.
In another aspect of the invention, a mobile also estimates a propagation delay to a neighbor base station and includes this as a parameter when calculating the appropriate time for measurements.
According to an exemplary embodiment, a base station site synchronized cellular communications system, wherein downlink control channel transmissions are allocated on discontinuous control channel carriers, includes a first base station, a neighboring base station, and a mobile station being served by the first base station. According to the embodiment, the mobile station includes a measurement unit which makes measurements of control channel signaling of the neighboring base station. Specifically, the measurement unit identifies a time slot during which the neighboring base station transmits control channel signaling, defines a duration of a control channel signaling measurement window to be less than a duration of the identified timeslot, and aligns the control channel signaling measurement window such that the measurements of the control channel signaling from the neighboring base station are made when the control channel signaling from the neighboring base station actually arrives at the measurement unit.
For example, the measurement unit can determine a position of a timeslot of the serving base station which corresponds with the identified timeslot of the neighboring base station, and then position the measurement window late in the timeslot of the serving base station. Alternatively, the measurement unit can determine a difference between a propagation delay from the serving base station and a propagation delay from the neighboring base station, and then adjust the position of the measurement window based on the determined difference. The mobile station can, for example, determine the difference between propagation delays by reading a synchronization burst of the neighboring base station.
According to other embodiments, the measurement unit can determine an absolute time reference of the serving base station, and then adjust the position of the measurement window based on the determined absolute time reference. Alternatively, the serving base station can transmit information regarding the relative geographic positions of the serving and neighboring base stations to the mobile station, and the measurement unit in the mobile station can adjust the position of the measurement window based on the transmitted information.